The present invention relates to a lithographic printing plate material from which a lithographic printing plate is made utilizing the silver complex diffusion transfer process and which is suitable for exposing to scanning type exposure light sources such as laser beam, especially red or infrared laser beam and red light emitting diode.
Lithographic printing plates which have transfer silver images which are obtained by the silver complex diffusion transfer process and which are utilized immediately as ink-receptive areas are disclosed in Japanese Patent Kokoku No. 48-30562 and Japanese Patent Kokai Nos. 53-21602, 54-10310, and 56-9750 and are well known.
A typical silver complex diffusion transfer process suitable for making such lithographic printing plates is carried out in the following manner. A photosensitive material which comprises a support and, provided thereon, an antihalation layer, a silver halide emulsion layer, and physical development nuclei layer is subjected to imagewise exposure and then to development treatment, whereby the silver halide of the latent image area formed by the imagewise exposure is converted to blackened silver in the emulsion layer and simultaneously, the silver halide other than the latent image area is dissolved by the action of silver halide complexing agent contained in the development processing solution and diffuses to the surface of the photosensitive material. The dissolved and diffused silver complex is precipitated as a silver image on the physical development nuclei of the surface layer by the reducing action of developing agent. If necessary, the silver image is subjected to lipophilization treatment after the development in order to enhance ink-receptivity of the resulting silver image. Thereafter, the resulting lithographic printing plate is mounted on an offset printing machine and inked image is transferred to a material on which the image is to be printed.
According to the above conventional method, the silver halide emulsion layer is spectrally sensitized with merocyanine dye, cyanine dye or the like so as to have a sensitive maximum in green region of around 550 nm and is subjected to an exposure for several seconds --several ten seconds by a process camera having a usual light source such as tungsten light source. However, the above conventional method has limit even though the above lithographic printing plate material is inherently excellent in sharpness and resolution. In addition, when a color print is to be obtained from a color original, not only the resolution is insufficient, but also production of the printing plate material and plate making operation are complicated.
For solving the above problems, there have been proposed methods of direct plate making using laser beam For example, lithographic printing plate materials applied to exposure to helium/neon laser, argon ion laser, semiconductor laser, light emitting diode and the like are disclosed in U.S. Pat. No. 4,501,811 and Japanese Patent Kokai Nos. 59-71055, 59-71056, 60-61752, 60-75838, 60-100148, 61-114235 and 63 47756.
However, as also mentioned in the above cited patent publications, in the case of lithographic printing plate materials which utilize silver complex diffusion transfer process, the surface physical development nuclei layer greatly affects the spectral sensitizability and as a result, sensitivity to the desired laser beam decreases or storage stability is deteriorated. Moreover, as a result of formation of soft transfer precipitated silver image by silver complex diffusion transfer processing, sharpness and resolution deteriorate and further stain in background occurs or the silver image is torn off during printing and so, a sufficient printing endurance cannot be obtained Thus, lithographic printing plate of high quality cannot be made. Therefore, not only the sensitizing dyes which can solve the above problems must have a sufficiently high sensitivity to the wavelength of laser beams, but also it is required that they are superior in storage stability, can form a hard transfer precipitated silver image, do not have bad effects such as stain of background and can form strong transfer precipitated silver image which is not torn off during printing even if it is silver of fine crystal.
On the other hand, as light sources for scanning plate making, there are used helium.cadmium laser, argon laser, helium.neon laser, semiconductor laser, light emitting diode, etc. Among them, red scanning exposure light sources of 600-700 nm such as helium.neon laser and red light emitting diode are generally often used in this field. Furthermore, high performance semiconductor lasers based on the recent progress of semiconductor technique has come to be used. The wavelength of semiconductor laser beam used as scanning exposure light sources is mainly 780 nm which is in near infrared region. In the case of helium.neon laser, oscillated wavelength is 632.8 nm and hence, in order to make the above-mentioned lithographic printing plate using helium.neon laser beam as a light source, the silver halide emulsion layer used in lithographic printing plate materials must satisfy the above various characteristics in addition to high sensitivity to 632.8 nm which is wavelength of helium.neon laser beam.
Such characteristics are also influenced by factors such as halogen composition of silver halide emulsion, size of silver halide grains and kind and degree of chemical sensitization. Emulsions mainly composed of silver chloride as a silver halide have the advantage that diffusion transfer development rapidly proceeds and transfer precipitated silver which is utilized as ink-receptive area can be readily produced. However, they are generally low in sensitivity and if they are sufficiently chemically sensitized for further enhancing sensitivity, the sensitivity to laser beam further decreases. Moreover, if the diffusion transfer development is carried out at too high speed, printing strength of the transfer precipitated silver tends to decrease.
In the case of a lithographic printing plate material from which a lithographic printing plate is made utilizing silver complex diffusion transfer process and which is suitable for exposing to scanning exposure light sources such as laser beams, especially red or infrared laser beam and red light emitting diode, it is difficult to satisfy all of the above requirements such as high sensitivity, high resolution, high printing endurance, storage stability, etc.
Furthermore, properties required for scanning type lithographic printing plate materials include image reproducibility and exposure latitude. In order to improve the image reproducibility and the exposure latitude, there have been employed various methods such as providing a black antihalation layer and addition of an irradiation inhibiting dye to emulsion, but these methods are still not satisfactory.